The conventional aluminium smelting technology which uses discontinuous prebaked anodes has major limitations in the areas of electrical energy efficiency, environmental pollution and worker health. Replacement of anodes contributes to low power efficiency and high fluoride emissions from pots, potrooms, butts processing areas and baking furnaces. Anode replacement involves a number of activities which are necessitated by the need to access the pots, remove spent anodes, add new anodes, cover these up, recover anode rods, cast iron and carbon from spent anodes, clean, crush and reprocess butts, return butt bath to the pots etc. All this adds to the cost of production and to environmental and health problems.
The conventional strategy used to deal with problems emanating from anode replacement has been to learn to live with them by alleviating their impact on worker health and safety and to reduce their cost through better economies of scale and increased mechanization. The aluminium industry has in the past developed butts cleaning technology and currently is looking for better ways of handling anodes, butts and bath and reducing pot emissions in potrooms. The underlying problem with this strategy is that no matter what is done with anode replacement and how this is done, no value is being added to the metal produced, or to any of the by-products of the process.
The discontinuous anode technology has impacted on the smelting technology in a number of ways. Cell design and construction, plant design, layout and capital infrastructure have all been affected. Apart from these, there are a number of jobs and operations which stem from anode replacement, all of which add to the cost, but not to the value of metal produced. These are: anode setting, butts handling, cleaning, crushing and grinding, bath crushing and handling, oreing-up of pots, anode rodding, fume treatment and others. Each of these steps and processes require significant capital investment and incur substantial operating costs.
The need to access the pots to replace anodes has meant that pots could not be adequately sealed. Excessive air flow rates are used to effectively purge the pots to keep the pot emissions in potlines down. During anode setting, the pots have to be opened and large volumes of fumes are released into the potroom atmosphere from open anode hole. Spent hot butts are often left in the potrooms to cool off before moving. Gaseous fluorides are produced by a reaction between the hot butts and moisture in the air which is drawn in from outside by the potroom and pot ventilation systems. This strategy of using large volumes of air to effectively purge the potlines and pots in order to keep the concentration of hazardous HF gas down, is doubly self-defeating. On the one hand, purging gas (atmospheric air) is the principal source of the hazard (HF production is directly proportional to the amount of moisture in the air), and on the other, the hazardous gas becomes so diluted that a very large and very efficient scrubbing system is required to achieve environmentally safe fluoride discharge levels.
Recycling of butts leads to introduction of fluoride salts into green anodes. These can react with hydrogen in the pitch binder during baking to give off HF. At higher temperatures, fluoride salts can also be vaporized. Fluorides contaminate the flue gas, react with refractories and accelerate the flue walls failures. Baking furnace flue gas is a major source of fluoride emissions which in future may require scrubbing.
Anode replacement has negative influence on the pot operations and its efficiency. A large mass of alumina and frozen crust falls into the bath during anode setting. Most of this alumina can not dissolve and ends up forming sludge. A freshly set cold anode chills off the bath, and this may cause the alumina being fed during the post setting period to remain undissolved due to lack of superheat. This forms additional sludge. The bath freezes on the anode surface preventing it from drawing current for several hours. This, not only increases the pot resistance, but causes current imbalance which may change the shape of metal pad profile and thus lead to a loss of current efficiency due to different anodes having different ACD's. All this limits the minimum voltage a cell can operate at and has a direct effect on its production efficiency and costs.
For all of the above reasons, the advantages to be gained by the use of continuous prebaked anodes deserve closer attention. Such advantages include:
(i) Lower capital costs through the elimination of the butts circuit and the rodding room facility. PA1 (ii) The production of high purity metal (99.9% Al) through the absence of recycled butts impurities. The iron level is, for example, expected to be below 0.03 wt %. PA1 (iii) The absence of butts impurities which will have a beneficial effect on the excess carbon consumption caused by air burn and carboxy reactivity. PA1 (iv) Increased life of baking oven flue walls resulting from the absence of corrosive bath components normally contained in recycled butts. PA1 (v) Lower bath losses because anode butts are not continually removed from the cell. PA1 (vi) Lower cell fluoride emissions and easier control of bath chemistry because the crust is broken less frequently. PA1 (vii) Decreased frequency of metal pad disturbances, because the regular setting of cold anodes is eliminated. PA1 (viii) More effective utilization of the total cathode area achieved by eliminating the center channel and employing larger anodes that span the width of the cell. PA1 (ix) Decreased effective current density by about 5-10%, through the elimination of "dead" anodes during cold anode changes. PA1 supplying electrical power to the cell, PA1 monitoring one or more operating parameters in the cell, and PA1 controlling the rate of heat extraction from the cell to maintain one or more of the operating parameters within set limits, wherein the rate of heat extraction from the cell can be controlled to permit operation of the cell at varying amperage. PA1 i) Use of off-peak electricity--the amperage may be varied on a daily basis to maximize metal production during off-peak periods when electricity prices are lower, thus decreasing the production cost of metal. PA1 ii) heat recovery and power co-generation--the heat recovered from the cell can be used to generate electricity, which may be used on-site or sold back to the electricity grid. Alternatively, the heated air could be used to produce steam, which could be used for power generation, bauxite digestion or sold to other users of steam located near the site. PA1 iii) the anode structure of the cell can act as a heat storage bank during off-peak, variable amperage operation. During high amperage operation of the cell at off-peak times, the extra heat generated can be at least partly used to increase the temperature of the anode support structure (although it will be realized that the temperature of the anodes and anode support structure should be maintained below a maximum level). As the anode support structure is a relatively massive structure, the increase in temperature absorbs a large quantity of energy. When the cell returns to lower amperage operation, this energy can be recovered by heat extraction to lower the temperature of the anode structure. The recovered heat can be used for co-generation of electricity, which may be sold to the electricity grid. This electricity is generated during peak periods and supplements the amount of power available on the grid. PA1 iv) Variable amperage operation enables the plant to optimise production efficiency by providing a way for cutting back production during down turns in demand and raising production during periods of high demand for the metal when the price is high. PA1 - anode temperature PA1 - anode support structure temperature PA1 - side wall temperature PA1 - frozen ledge thickness PA1 - bath temperature
Continuous prebaked anode technology first appeared in the early 1960's. A number of problem areas limited the effectiveness of the technology. Early operations were plagued with anode separation problems due to glue failures, although this has since largely been resolved. The current feeder technology was based on the horizontal stud Soderberg concept in which current was conducted to the anodes by four steel studs pressed into the ends of the anode at a sloping angle of about 20.degree. downward to allow for quicker fast and better electrical contact. However, this created strong vertical magnetic fields in the cell and aggravated the already existing bad magnetic design of the end-to-end cells. Absence of effective anode insulation and cover led to high top heat losses and significant anode airburn. In spite of these shortcomings, the continuous prebaked technology was able to demonstrate the advantages of increased metal purity, improved environmental control, reduced bath material consumption and reduced labor requirement mentioned above.
Other designs for continuous pre-baked anodes have also been described. Australian Patent Application No. 48715/90, by Norsk Hydro A. S. describes an aluminium electrolysis cell having a continuous anode. The anode is divided into a number of easily detachable cassettes or holders which provide for continuous feeding of carbon anodes. Additional cassettes containing equipment for the of additives such as alumina, to the bath are located between the anode-holding cassettes. The cassettes have projections located on their upper portion and these projections rest on vertically movable bars to thereby support the cassettes.
The construction of a preferred form of the cassettes is shown in FIGS. 3-5 of the patent. Each cassette includes an upper part having a guide for the carbon anodes. The lower part of the guides comprises a holder arrangement in the form of a clamping device connected to the upper parts of the guides by means of elongate stays. The clamping arrangement and associated stay are located at each corner of the carbon anode and do not extend completely around the periphery of the carbon anode. The holder arrangement holds the stack of carbon blocks by means of frictional force. The holder arrangement also conducts electricity to the anode carbon.
The clamping devices on each corner of the anode block are connected to each other by cross stays. Swallow tail grooves are placed along the long side of the anodes in order to provide extra electrical current contacts to improve current distribution in the anode. Force is applied to the clamping means by way of lifting intermediate stays which acts to bend the cross stays and pull the clamping arrangements on each corner closer together.
In a preferred embodiment, the cassettes are provided with cooling conduits to reduce the temperature in the cassette walls. As shown in FIG. 5 of the patent, the clamping arrangement and associated stays are provided with bores or conduits to allow the circulation of a cooling fluid therein.
The arrangement described in AU,A,48715/90 provides clamping members located only at the corners of the anode blocks. As a result, large surfaces of the anode carbon are exposed which causes considerable potential for anode burn. Further, as the clamping members provide electrical contact for the anode carbon, current distribution in the anode is not optimal. The clamping members are capable of being cooled by a cooling fluid to control the temperature in the cassette walls. However, substantially no heat is recovered from the surfaces of the anode carbon that do not contact the clamping means, and this represents a loss of heat. The anode structure is also made from a number of separate cassettes, which increases the complexity and cost of fabrication of the anode structure. If cooling is provided, the clamping means must also include conduits or bores, which further adds to the complexity and cost of the anode structure.
U.S. Pat. No. 2,958,641, assigned to Renyolds Metals Company, describes an anode "bundle" for use in aluminium electrolysis cells. The anode "bundle" includes a pack of pre-baked carbon slabs interleaved above their lower ends with steel plates. The bundle of slabs and plates are secured by a clamping means. The anode block is described as having a service life of between 30 and 60 days and is not used as a continuous anode. Indeed, each anode includes anode cap assemblies connected to the top thereof and such cap assemblies would preclude operation of the anode as a continuous anode. Furthermore, large areas of the anode surface are exposed to the atmosphere and the potential for anode burn is accordingly high.
Several patents and literature articles have also discussed heat recovery from aluminium electrolysis cells. In this regard, U.S. Pat. Nos. 4,608,134 and 4,608,135 describe cooling of the side wall of an electrolysis cell at a site adjacent the surface of the molten bath to promote the formation of a protective layer of frozen bath over the side wall adjacent the cooling means. The heat recovered from the side wall is subsequently returned to the cell. Both of these patents are concerned with prevention of freeze-up of low power cells that are operated at substantially constant power inputs. Except for the cooling means included in the side wall, the cells are essentially conventional in design. An article in "Light Metals", 1983, by P. H. Dekloff, entitled "Heat Recovery from Pot Gas from Electrolytic Reduction Cells for Producing Aluminium" describes the recovery of energy from the off-gas from aluminium smelting cells. Heat from the smelting cells is lost to the off-gas by passive heat transfer and subsequently recovered remote from the cell. The overall heat balance of the cell is not affected by the recovery of heat from the off-gas.